[1] In transient landscapes, adjustments in river channel width, roughness, and alluvial cover, in addition to slope, provide potentially important but poorly understood mechanisms by which bedrock channels accommodate changes in external forcing. We used a laboratory flume to investigate experimentally how bedrock channel slope, width, roughness, alluvial cover, and incision rate collectively adjusted during the transient incision of an initially smooth channel with a varying bed load supply rate. When the channel was free of alluvial cover, incision was focused over a fraction of the bed width that varied strongly with both bed load supply and bed load transport capacity. Nondimensionalization yields a relationship for the width of active incision that explicitly incorporates bed load supply rate, sediment grain size, and bed shear stress, which suggests that in natural channels, width may respond dynamically to accommodate changes in bed load sediment supply. Because increases in sediment supply widened the band of active bed load sediment transport and thus the width over which incision took place, mass removal from the bed scaled with sediment supply when the bed was free of cover, consistent with incision being limited by the availability of erosive tools. However, bed roughness growth due to the spatial variation of incision during the experiment eventually inhibited bed load transport efficiency. This, in turn, led to deposition of alluvial cover and the suppression of incision on the bed at high sediment supply rates, consistent with incision being limited by the extent of alluvial cover deposited on the bed. The dynamics of roughness creation and alluvial cover deposition can therefore drive both negative and positive feedbacks on incision rate change following sediment supply perturbations. These experimental results offer several potentially field-testable hypotheses that together may help explain variability in the width, slope, and bed roughness of bedrock river channels in transient landscapes.Citation: Finnegan, N. J., L. S. Sklar, and T. K. Fuller (2007), Interplay of sediment supply, river incision, and channel morphology revealed by the transient evolution of an experimental bedrock channel,

Along the South Fork of the Eel River in northern California, paleoerosion rates derived from 10 Be concentrations in late Pleistocene strath terrace sediment are a factor of 2 greater than erosion rates derived from modern stream sediment and 3.5 times greater than paleoerosion rates from the Pleistocene-Holocene transition. Using these results as a proxy for sediment supply, we provide quantitative fi eld-based evidence that extensive strath planation is linked to elevated sediment supply conditions. We have used optically stimulated luminescence (OSL) to date strath terrace sediment and fi nd that the highest erosion rates and most extensive period of strath planation occurred during a period of increased precipitation in the late Pleistocene. Based on our OSL data, we estimate that bedrock channel lowering rates have outpaced basin-averaged erosion rates by a factor of three since abandonment of the extensive late Pleistocene strath surface. Thus, our data indicate that hillslope relief has been increasing for the past ~20 ka.

Physical experiments were conducted to evaluate the efficacy of bed load particle impacts as a mechanism of lateral bedrock erosion. In addition, we explored how changes in channel bed roughness, as would occur during development of an alluvial cover, influence rates of lateral erosion. Experimental channels were constructed to have erodible walls and a nonerodible bed using different mixtures of sand and cement. Bed roughness was varied along the length of the channel by embedding sediment particles of different size in the channel bed mixture. Lateral wall erosion from clear‐water flow was negligible. Lateral erosion during periods in which bed load was supplied to the channel removed as much as 3% of the initial wetted cross‐sectional area. The vertical distribution of erosion was limited to the base of the channel wall, producing channels with undercut banks. The addition of roughness elements to an otherwise smooth bed caused rates of lateral erosion to increase by as much as a factor of 7 during periods of bed load supply. However, a minimum roughness element diameter of approximately half the median bed load particle diameter was required before a substantial increase in erosion was observed. Beyond this minimum threshold size, further increases in the relative size of roughness elements did not substantially change the rate of wall erosion despite changes in total boundary shear stress. The deflection of saltating bed load particles into the channel wall by fixed roughness elements is hypothesized to be the driver of the observed increase in lateral erosion rates.

Bedrock incision plays a key role in determining the pace of landscape evolution. Much is known about how bedrock rivers incise vertically, but less is known about lateral erosion. Lateral erosion is widely thought to occur when the bed is alluviated, which prevents vertical erosion and deflects the downstream transport of bedload particles into channel walls. Here we develop a model for lateral erosion by bedload particle impacts. The lateral erosion rate is the product of the volume eroded per particle impact and the impact rate. The volume eroded per particle impact is modeled by tracking the motion of bedload particles from collision with roughness elements to impacts on the wall. The impact rate on the wall is calculated from deflection rates on roughness elements. The numerical model further incorporates the coevolution of wall morphology, shear stress, and erosion rate. The model predicts the undercut wall shape observed in physical experiments. The nondimensional lateral erosion rate is used to explore how lateral erosion varies under different relative sediment supply (ratio of supply to transport capacity) and transport stage conditions. Maximum lateral erosion rates occur at high relative sediment supply rates (~0.7) and moderate transport stages (~10). The competition between lateral and vertical erosion is investigated by coupling the saltation‐abrasion vertical erosion model with our lateral erosion model. The results suggest that vertical erosion dominates under near 75% of supply and transport stage conditions but is outpaced by lateral erosion near the threshold for full bed coverage.

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